Production of xylenes



G. F'UHCIELL ETAL.

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PRODUCTION OF XYLENES 2 Sheets-Sheet 2 Filed May 20, 1.947

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Gienn Puree Douglas C. Hansel@ @y "wir AQQWMQMMEM Patented Jan. 31, 1950 PRODUCTION OF XYLENES Glenn Purcell, Hillsdale, N. J., and Douglas C. Hausch, Jackson Heights, N. Y., asslgnors to Shell Development Company, San Francisco, Calif., a corporation of Delaware Application May 20, 1947, Serial No. 749,328

7 Claims. l

This invention relates to the production of aro# matic hydrocarbons, and particularly of xylenes. It is more particularly concerned with an improved process for the production and recovery of aromatic hydrocarbons, more specifically of xylenes of high purity, from naphthenes and/or naphthene-forming substances.

It is known that certain narrow-boiling petroleum fractions, in some cases virgin petroleum fractions and in other cases fractions obtained by catalytic conversions such as destructive hydrogenation to convert certain parafinic hydrocarbons to cycloparafflnic hydrocarbons, contain relatively large proportions of naphthenic (cycloaliphatic) hydrocarbons. such as cyclohexane, methylcyclohexane, and dimethylcyclohexanes. It is also known to subject such narrow-boiling fractions to conversion conditions such as catalytic reforming or dehydrogenation and thereby to convert the cyclohexanes to corresponding aromatic hydrocarbons. It has been proposed to recover the thus formed aromatic hydrocarbons from unconverted cyclohexanes and associated open chain paraflinic hydrocarbons (that is, noncylic paraflinic hydrocarbons) by liquid-liquid solvent extraction with selective solvents for aromatic hydrocarbons, such as liquid sulfur dioxide, furfural, phenol, alkyl phenols. etc., or by liquid-vapor extraction (extractive distillation) using those selective solvents which have boiling points sufliciently higher than the bubble or boiling temperature of the mixture of hydrocarbons being thus treated, so as not to be carried over in the raffinate to any appreciable extent.

However, it has now been found that the use of phenols, in general, and of phenol itself in particular, when used in accordance with the above-indicated suggestions of the prior art is not entirely satisfactory. Thus, when a phenol is utilized as the selective solvent in an extractive distillation process for the separation of aromatic hydrocarbons from a mixture thereof with unchanged cyclohexanes from which they were obtained by dehydrogenation, with or without paraffin hydrocarbons of relatively similar boiling points, the amount of the phenol carried overhead in the raflinate is large enough to make the process too uneconomical on account of this loss of phenol.

It is a principal object, therefore, of this invention to provide an improved process for the production and recovery of aromatic hydrocarbons. A further object is to provide an improved process for the separation of aromatic hydrocar- (Cl. 26o-668) bons from non-aromatic hydrocarbons of relatively similar volatility characteristics. Another object is to provide an improved process for the separation of aromatic hydrocarbons from nonaromatic paraflinic and cycloaliphatic hydrocarbons often associated therewith. A more specinc object is to produce and recover xylenes from dimethylcyclohexane in high yield and purity in a more economical manner than has been available heretofore. A still more specific object is to provide an improved process for recovering xylenes from admixture with dimethylcyclohexane and similar boiling open chain paramn hydrocarbons. These and other objects will be more evident from the following description of the invention.

Now, in accordance with the present invention, an improved process is provided for the production of aromatic hydrocarbons which comprises: 1) dehydrogenating a narrow-boiling range hydrocarbon mixture which contains predominately cyclohexane, methylcyciohexane or a dimethylcyclohexane, that is, cycloaliphatic hydrocarbons which correspond in carbon skeleton to a desired aromatic hydrocarbon, thereby producing a mixture containing unchanged a cycloaliphatic hydrocarbon, corresponding aromatic hydrocarbons, and with or without other constituents such as paraiiins, i. e. non-cyclic aliphatic or paraflinic hydrocarbons, etc., (2) fractionally distilling the resulting mixture to remove essentially all of the cyclohexane or alkylcyclohexane (cyclo-aliphatic hydrocarbon) present therein, and (3) extracting the resulting aromatic fraction in the vapor phase with a. phenol or a phenol-containing selective solvent in the liquid phase to separate the aromatic hydrocar bons from the non-aromatic hydrocarbons ad mixed therewith. By phenol-containing is meant, e. g., phenol plus a polar modifier, such as 240% water.

Described more specifically with reference to an exemplary application of the invention. namely, to the production and recovery of xylenes, the invention provides a process wherein: 1) a virgin petroleum oil rich in dimethylcyclohexane or a destructively hydrogenated petroleum fraction containing dimethylcyclohexane, is fractionated as is known in the art to produce a relatively narrow-boiling fraction with a boiling range of about 240-270 F. and which is rich in dimethylcyclohexane; the resulting dimethylcyclohexane concentrate is subjected to catalytic reforming (dehydrogenation) for the purpose of converting the dimethylcyclohexanes to xylenes; the resulting mixture of xylenes, unchanged cyclohexanes and any parafn hydrocarbons present therein is carefully fractionated to remove from the xylenes the naphthenes such as dimethylcyclohexane and separating the xylenescontaining fraction boiling between 260 F. and 300 F.; and subjecting the resulting xylenes fraction to a liquid-vapor extraction with a phenol to extract the xylenes from the non-aromatic (principally open chain parafnic) hydrocarbons; and recovering the xylenes from the phenol extract.

The nature of the process of this invention and the manner in which it is carried out will be fully understood from the following description of a preferred embodiment thereof taken with reference to the accompanying dravnng which is made a part of the specification wherein Fig. I is a diagrammatic sketch illustrating the production of xylenes from a hydrocarbon fractionl which contains cyclohexanes, and Fig. II is a graph showing the inuence of naphthenes on the loss of phenol in the raffinate of a distillation process.

Referring to Fig. I of the drawing, a hydrocarbon fraction containing cyclohexanes, including dimethylcyclohexanes, is delivered through line II to a fractionating column I2, wherein it is fractionated to remove through a line I4 all hydrocarbons which boil below 240 F. at atmospheric pressure, as an end point (E. PJ, and the remaining part is transferred through a line I5 to a fractionating column i6, wherein the material boiling between 240 F. and 270 F. under normal atmospheric pressure is separated, by removing as Vapor through line I1, from the residual portion boiling above 270 F. which is withdrawn through a line I8. This fractionation serves, as will be seen from the data in Table I, to separate the desired dimethylcyclohexanes from both (1) the lower-boiling, natural-occurring benzene and toluene, lower-boiling cyclohexane and methylcyclohexane and the lowerboiling normaland iso-parain hydrocarbons, and (2) the higher-boiling ethyl benzene, xylenes and paraffin hydrocarbons.

The above-described fractionations may be carried out under various coordinated conditions of temperature and pressure and the fractionating columns may be of usual bubble-cap plate type construction or any other suitable type as desired.

The narrow-boiling fraction of dimethylcyclohexanes thus separated by the two fractionations in fractionators I2 and I6 is next delivered through lines I1 and I9 to a dehydrogenation zone 20 wherein it is at least partially dehydrogenated to a mixture of xylenes. The dehydrogenation zone is adapted for dehydrogenation of the methylcyclohexanes to xylenes and it contains a catalytic material which promotes catalytic reforming or dehydrogenation.

In dehydrogenation zone 20. the fraction boiling between 240 F. and 270 F. is subjected to catalytic dehydrogenation for the purpose off 'dehydrogenation zone at a rate of between 0.3 and 3 volumes of liquid oil per volume of catalyst per hour. The catalytic material used in dehydrogenation zone 20 may consist of a major amount of aluminum oxide in any of its various forms together with a minor amount of an oxide and/or sulde of a metal of the II, IV, V, VI or VIH groups of the periodic system, a mixture of tungsten and nickel suldes being especially eiective.

Products of the reaction in dehydrogenation zone 20 are carried by line 2I to a fractionator 22, which is provided with cooler 24 and reboiler means 25. The lighter materials boiling up to 260 F. at atmospheric pressure, including unchanged dimethylcyclohexanes, are removed overhead through line 26, all or a portion thereof, as desired, being recycled by way o! line 29 to the feed for the dehydrogenation zone 20 and thus subjected to further dehydrogenation followed by the straight distillation and extractive distillation already described. Instead of recycling the naphthenes, the overhead naphthene fraction may be led to another dehydrogenation zone, dehydrogenated therein to produce a further amount of aromatics, and the resulting lproduct then subjected to extractive distillation.

preceded, if desirable, by a straight fractional distillation, to recover the aromatic content thereof. The fraction boiling above 260 F. is delivered through a line 30 to a fractionator 3I wherein the portion thereof boiling between 260 F. and 300 F. is separated as overhead through line 32, from the heavier part boiling at 300 F. and higher, which is withdrawn through a line 34.

The 26o-300 F. fraction removed from fractionator 3I in line 32 is introduced into an intermediate section of a tower 35 adapted for counter-current liquid-vapor ilow and provided with a reboiler means 38. The 260-300 F. fraction (consisting essentially of xylenes and open-chain paraflin hydrocarbons of similar boiling range) is countercurrently contacted in tower 35 at a temperature at or slightly above the normal bubble temperature of said fraction, in accordance with the practices of extractive distillation, with phenol being introduced into the tower through a line 38. The non-aromatic hydrocarbons are removed as overhead vapor through line 31 and the aromatic hydrocarbons, the xylenes, are withdrawn through line 33 as a phenolxylenes extract. The phenol-xylenes extract is fractionated in fractionating column 40 with the separation of the xylenes as overhead through line 4I, condenser 42 and line 44 with partial return as reflux through line 45. The thus separated phenol is withdrawn through a line 46 and returned by means of line 33 to the extractive distillation zone in tower 35.

Application of the above-described coordinated combination of steps has resulted in the production and separation of xylenes in high yield and amants high recovery with high purity and at the same time with a substantial reduction in loss of the extractive solvent phenol to the raiinate from the extractive distillation operations. This latter advantage resulting from practice of the invention appears to be attributable, at least in a large degree, to the separation from the reaction product taken from the dehydrogenatlon zone, of the naphthenes, e. g. unconverted dimethylcyclohexanes, present therein and which are more volatile than the xylenes, prior to the liquidvapor extraction step.

The inuence of naphthenes on the loss of a phenol in the railinate (overhead) from the distillation of an aromatic-containing hydrocarbon mixture containing a phenol is shown by the data in Table II. which data were obtained by a precision fractional distillation of a xylenes concentrate, obtained directly from the dehydrogenation zone (Fig. I), to which concentrate had been added grams of phenol for each 100 cc. of xylene concentrate. The more complete distillation data are shown in the graph of Fig. II.

TABLE II Fractionaton of synthetic :c1/Iene concentrate plus 10 grams phenol/100 cc. concentrate Composition o! Dislilizxlo Fraction Boiling 21H20 'W The phenol distribution shown in Fig. II and indicated in Table II indicates that a substantial amount of phenol distills over with raiilnate which contains naphthenes, but that material boiling above dimethylcyclohexanes (approximately 260 F. at normal pressure), which consists of open chain parafns and aromatics, can be separated from phenol by fractionation without excessive loss of the phenol in the distillate. In the practice of the present invention, therefore, the xylene concentrate is rst distilled to remove any accompanying lighter naphthenes, such as dimethylcyclohexanes, as by distilling ofi the fraction which boils up to about 260 F. normal pressure), even preferably up to 26l265 F., or up to within about 10-15 F. of the boiling point of the lowest boiling desired aromatic prior to the recovery of the xylenes from parailins and higher naphthenes by liquid-vapor extraction with a phenol. Thus, the xylenes-concentrate to be extractively distilled should have a minimum boiling point of about 260 F., and preferably 261265 F.

In the production and recovery of xylenes and other aromatic hydrocarbons by practicing this invention, various phenols, i. e. cresols, xylenols and other alkyl phenols may be utilized. In particular, for aromatics of higher molecular weight than the xylenes, alkyl phenols may be utilized and the naphthenes of corresponding carbon skeleton are separated from the aromatic hydrocarbons, prior to extractive distillation, by fractional distillation to remove all material which boils up to approximately the boiling point of the aromatic hydrocarbons, or up to within about l0l5 F. of the boiling point of the aromatic hydrocarbons.

It will be understood further, that in the recovery of xylenes from mixtures containing them together with naphthenes in general, at least some of which naphthenes have boiling temperatures appreciably below the boiling temperature of the desired aromatic hydrocarbon and which lower boiling naphthenes are substantially removed by a pre-fractionation, the alkyl phenols may be used with advantage over phenol itself in that the loss of phenol in the raminate can be still further reduced. Also. the invention is applicable to the recovery oi aromatics from naphthenes of slightly lower boiling points than the aromatics whether or not they have the same carbon skeleton as that of the aromatica.

We claim as our invention:

l. A process for the production cf a xylene which comprises the steps of: subjecting a hydrocarbon mixture consisting essentially of a predominant proportion of a. dimethylcyclohexane hydrocarbon and oi open chain paraffin hydrocarbons with similar volatility to said dimethyl cyclohexane hydrocarbon and to a xylene of similar carbon skeleton to said cyclohexane to dehydrogenation under conditions to dehydrogenate at least a substantial portion of said dimethylcyclohexane hydrocarbon to said xylene of similar carbon skeleton while leaving a substantial portion unchanged, thereby producing a second hydrocarbon mixture consisting essentially o1' said xylene. unchanged dimethylcyclohexane and said parailln hydrocarbons; fractionally distilling said second hydrocarbon mixture to remove the portion of said second mixture which is removed at a. temperature up to and including 260 F. at normal atmospheric pressure, to thereby remove essentially all of the unchanged dimethylcyclohexane; subjecting the remaining portion o! the second mixture to liquid-vapor extraction with phenol at a temperature above the bubble temperature of said remaining portion to separate said xylene as an extract in said phenol from the paraiiln hydrocarbons present therein which have similar volatility to said xylene; and recovering the xylene from said extract.

2. A `process for the production oi xylenes which comprises the steps of: subjecting e. hydrocarbon fraction consisting essentially of a predominant proportion oi' dimethylcyclohexane hydrocarbons and of open chain parailln hydrocarbons with similar volatility to said dimethylcyclohexanes and to xylenes, to catalytic dehydrogenation under conditions to dehydrlogenate at least a substantial portion of said dimethylcyclohexanes to corresponding xylenes while leaving a substantial portion of unchanged dimethylcyciohexanes, thereby producing a mixture consisting essentially of xylenes, unchanged dimethylcyclohexanes and said parailin hydro carbons; fractionally distilling essentially all of the unchanged dimethylcyclohexanes from the xylenes of said mixture; subjecting the remaining mixture to liquid-vapor extraction with a phenol at a temperature above the bubble temperature of said remaining mixture to separate said xylenes as an extract in said phenol from the parafllnic hydrocarbons, with a minimum loss of said phenol with the parafilnic hydrocar bons in the overhead from said liquid-vapor extraction; and recovering the aromatic hydrocarbons from said extract.

3. A process for the production of an aromatic hydrocarbon which comprises the steps of: subjecting a hydrocarbon fraction, consisting essentially of a predominant proportion of a cycloaliphatic hydrocarbon corresponding in carbon skeleton to a desired aromatic hydrocarbon and of open-chain paramn hydrocarbons, to dehydrogenation under conditions to dehydrogenate at least a substantial portion of said cycloaliphatic hydrocarbon to the corresponding aromatic hydrocarbon while leaving a substantial portion of unchanged cycloaliphatic hydrocarbon, thereby producing a hydrocarbon mixture consisting essentially of said aromatic hydrocarbon, unchanged cycloaliphatic hydrocarbon and said parailln hydrocarbons; fractionally distilling said hydrocarbon mixture to remove as overhead the portion oi said mixture which is removed at a temperature belowand up to within -15 F. of the boiling temperature of said aromatic hydrocarbon, to thereby remove essentially all of the unchanged cycloaliphatic hydrocarbon; extractively distilling the remaining portion of the mixture with a phenol to separate said aromatic hydrocarbon as an extract in said phenol from the paraffin hydrocarbons; and recovering the Y aromatic hydrocarbons from said extract.

4. The process of claim 3, wherein the fractionally distilled unchanged cycloaiiphatic hydrocarbon is recycled to the dehydrogenation step to be subjected to a further dehydrogenation together with another portion of a hydrocarbon fraction corresponding to the rst named hydro-- carbon fraction as dened in claim 3.

5. The process of claim 3, wherein the fracunder conditions to dehydrogenate a substantial portion oi' it to the corresponding aromatic hydrocarbon, and the product resulting from said tionally distilled unchanged cycloaliphatic hydrocarbon is subjected to a further dehydrogenation :further dehydrogenation is subjected to a sequence of fractional distillation and extractive distillation as dened in claim 3.

6. A process for the production of xylenes which comprises the steps of z subjecting a hydrocarbon fraction consisting essentially of a predominant proportion of dimethylcyclohexane hydrocarbons and of open-chain paraiiin hydrocarbons with similar volatility to said dimethyl cyclohexanes, which fraction has a boiling range of from about 240 F. to about 270 F.; to dehydrogenation under conditions to dehydrogenate at least a substantial portion of said dimethylcyclohexane hydrocarbons to the corresponding xylenes while leaving a substantial portion of unchanged dimethylcyclohexanes, thereby producing a mixture consisting essentially oi xylenes, unchanged dimethylcyclohexanes and said paraffin hydrocarbons; fractionally distilling said mixture to remove as overhead distillate the portion thereof which is removed at a temperature up to 265 F. at normal atmospheric pressure, thus removing in said overhead distillate essentially all of said unchanged dimethylcyclohexanes,

and recovering a xylenes fraction substantially free from dimethylcyclohexane; combining said overhead distillate with another portion of hydrocarbon fraction as nrst above described and subjecting the resulting mixture to the foregoing described operations; extractively distilling the recovered xylenes fraction with a phenol to separate said xylenes as an extract in said phenol from the paramn hydrocarbons, with a minimum loss of said phenol with the parailln hydrocarbons in the overhead from said extractive distillation; and recovering the aromatic hydrocarbons from said extract.

7. In a process for the production of xylenes which comprises the steps of subjecting a hydrocarbon fraction consisting essentially of a predominant proportion of dimethylcyclohexane hydrocarbons and of open-chain paramn hydrocarbons with similar volatility to said dimethylcyclohexanes and to xylenes, to catalytic dehydrogenation under conditions to dehydrogenate at least a substantial portion of said dimethylcyclohexanes to corresponding xylenes, while leavinga substantial portion of unchanged dimethylcyclohexanes, thereby producing a mixture consisting essentially of xylenes, unchanged dimethylcyclohexanes and said parafiln hydrocarbons; fractionally distilling said mixture and separating therefrom a xylenes fraction containing essentially all of said xylenes in said mixture; subjecting said separated xylenes fraction to extractive distillation with a phenol at a temperature above the bubble temperature of said xylenes fraction to separate said xylenes as an extract in said phenol from the non-aromatic hydrocarbons in said xylenes fraction; and recovering the xylenes from said extract; the improvement which comprises carrying out the fractional distillation oi the mixture consisting essentially of xylenes, unchanged dimethylcyclohexanes and paraflln hydrocarbons at a sulciently high temperature so as to remove essentially all of the unchanged dimethylcyclohexanes from the xylenes and to separate therefrom a xylenes fraction which is essentially free from dimethylcyclohexanes. l

GLENN PURCELL. DOUGLAS C. HAUSCH.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,288,126 Dunn et al June 30, 1942 2,375,573 Meier May 8, 1945 2,392,749 Lewis et al. Jan. 8, 1946 2,396,761 Tildon Mar. 19, 1946 

7. IN A PROCESS FOR THE PRODUCTION OF XYLENES WHICH COMPRISES THE STEPS OF: SUBJECTING A HYDROCARBON FRACTION CONSISTING ESSENTIALLY OF A PREDOMINANT PROPORTION OF DIMETHYLCYCLOHEXANE HYDROCARBONS AND OF OPEN-CHAIN PARAFFIN HYDROCARBONS WITH SIMILAR VOLATILITY TO SAID DIMETHYLCYCLOHEXANES AND TO XYLENES, TO CATALYTIC DEHYDROGENATION UNDER CONDITIONS TO DEHYDROGENATE AT LEAST A SUBSTANTIAL PORTION OF SAID DIMETHYLCYCLOHEXANES TO CORRESPONDING XYLENES, WHILE LEAVING A SUBSTANTIAL PORTION OF UNCHANGED DIMETHYLCYCLOHEXANES, THEREBY PRODUCING A MIXTURE CONSISTING ESSENTIALLY OF XYLENES, UNCHANGED DIMETHYLCYCLOHEXANES AND SAID PARAFFIN HYDROCARBONS; FRACTIONALLY DISTILLING SAID MIXTURE AND SEPARATING THEREFROM A XYLENES FRACTION CONTAINING ESSENTIALLY ALL OF SAID XYLENES IN SAID MIXTURE; SUBJECTING SAID SEPARATED XYLENES FRACTION TO EXTRACTIVE DISTILLATION WITH A PHENOL AT A TEMPERATURE ABOVE THE BUBBLE TEMPERATURE OF SAID XYLENES FRACTION TO SEPARATE SAID XYLENES AS AN EXTRACT IN SAID PHENOL FROM THE NON-AROMATIC HYDROCARBONS IN SAID XYLENES FRACTION; AND RECOVERING THE XYLENES FROM SAID EXTRACT; THE IMPROVEMENT WHICH COMPRISES CARRYING OUT THE FRACTIONAL DISTILLATION OF THE MIXTURE CONSISTING ESSENTIALLY OF XYLENES, UNCHANGED DIMETHYLCYCLOHEXANES AND PARAFFIN HYDROCARBONS AT A SUFFICIENTLY HIGH TEMPERATURE SO AS TO REMOVE ESSENTIALLY ALL OF THE UNCHANGED DIMETHYLCYCLOHEXANES FROM THE XYLENES AND TO SEPARATE THEREFROM A XYLENES FRACTION WHICH IS ESSENTIALLY FREE FROM DIMETHYLCYCLOHEXANES. 